Method of treatment of ferrous metal parts to increase their resistance to wear and seizure

ABSTRACT

A method of surface-hardening mechanical parts of ferrous metals by introducing into the surface layers of the treated parts a plurality of elements chosen from the groups consisting of nitrogen and carbon; nitrogen, carbon and sulphur, nitrogen, carbon, sulphur and phosphorus; in which said parts to be treated are immersed in a bath of molten salts at a temperature of 450* to 600*C., said bath being of the kind comprising CO3 and CNO ions on the one hand and on the other hand alkaline ions comprising Li , K and Na ions. The bath may further comprise sulphur anions chosen from the group consisting of S , SO3 and S2O3 , and also PO3 phosphorus ions. A regenerating agent comprising urea is further introduced into said bath in order to convert excess CO3 ions to CNO ions, and an apparatus is described for carrying this method into effect.

United States Patent 1 1 1111 3,912,547

Gaucher et al. Oct. 14, 1975 [54] METHOD OF TREATIVIENT OF FERROUS 2,553,865 5/1951 Newell..... 148/155 X METAL PARTS TO INCREASE THEIR /1323 RESISTANCE TO WEAR AND SEIZURE 3Z3O3IO63 2/1967 [75] Inventors; Antoine Gaucher, St E[ienne; 3,331,708 7/1967 Buitkus 148/ 15.5

Ge'rard Guilhot, St-Jean Bonnefonds, both of France Primary ExaminerRalph S. Kendall [73] Assignee: Centre Stephanois de Recherches Asslstam Exammer charles 'wolfe Mecaniques Hydmmecanique et Attorney, Agent, or F1rm-Young & Thompson Frottement, Andrezieux-Boutheon, Fra [57] ABSTRACT [22] Filed: Feb. 8 1973 A method of surface-hardening mechanical parts of ferrous metals by introducing into the surface layers of PP 330,646 the treated parts a plurality of elements chosen from the groups consisting of nitrogen and carbon; nitro- [30] Foreign Application Priority Data gen, carbon and sulphur, nitrogen, carbon, sulphur F b 18 1972 F 72 05498 and phosphorus; in which said parts to be treated are 1972 France 72'42350 immersed in a bath of molten salts at a temperature of l973 France 72-02212 450 to 600c., said bath being of the kind comprising an. rance and ions on the one hand and on the other hand alkaline ions comprising Li, K and Na 148/ ions. The bath may further comprise sulphur anions chosen from the group Consisting of and [58] Field of Search 148/ 15.5, 15, 20, 6.11 and also PO; phosphorus ions A regenerat ing agent comprising urea is further introduced into [56] References Clted said bath in order to convert excess CO ions to UNITED STATES PATENTS CNO ions, and an apparatus is described for carrying 2,041,769 5/1936 Larkin 148/155 X this method into effect,

2,049,806 8/1936 Holt 2,254,296 9/1941 Kramer 148 155 7 Chums, 1 Drawing Flgure METHOD OF TREATMENT OF FERROUS METAL PARTS TO INCREASE THEIR RESISTANCE TO WEAR AND SEIZURE Treatments are already known in baths of molten salts which introduce into the superficial layers of mechanical parts, especially of ferrous metals, either nitrogen and carbon or nitrogen, carbon and sulphur, for the particular purpose of improving their resistance to wear and to seizure.

All these treatments of carbo-nitridation and carbo-nitro-sulphuration produce, at the surface of the treated ferrous parts, a white micro-layer having a base mainly of nitrides and carbides of iron in the case of carbo-nitridation, or of nitrides, carbides and sulphides of iron in the case of carbo-nitro-sulphuration.

This white micro-layer is especially intended to increase the resistance to wear of the treated parts. This resistance to wear varies according to the thickness and the compactness of this white micro-layer, and in consequence it would be advantageous to be able to produce white non-porous micro-layers having the greatest possible thickness.

For a given temperature of treatment, (generally comprised between 500 and 600C), the thickness of the white micro-layer is, at least at the beginning, a function of the time of treatment. The maximum useful thickness which is currently obtained with all the known treatments is microns in the case where the part treated is of unalloyed construction steel, the carbon content of which is 0.35% at the end of 1% to 2 hours immersion; beyond this period of immersion, the white micro-layer becomes porous and friable, which reduces the resistance to wear of the parts treated.

Work carried out by the Applicants has made it possible to find that the presence of the cation Li", particularly in well defined proportions in a bath of carbonitriding or carbo-nitro-sulphurizing molten salts, makes it possible to obtain, for the same temperature and the same treatment time, white micro-layers which are not porous and which are thicker than those obtained in the baths proposed up to the present time. In addition, the presence of the cation Li permits good chemical stability of the bath to be obtained, which is added to its anti-corrosion function.

Work carried out by the Applicants has also made it possible to establish that the cation Li if it is utilized alone, does not give the treatment bath sufficient fluidity and that suitable fluidity is obtained when at least one of the ions K and Na is associated with the Li ion.

The present invention has for its object a method of treatment of ferrous metal parts having the effect of introducing into the superficial layers of the parts treated the elements nitrogen and carbon, or nitrogen, carbon and sulphur, or alternatively, nitrogen, carbon, sulphur and phosphorus, and consisting of immersing the parts to be treated in a bath of molten salts, in which the bath is of the type comprising on the one hand CO and CNO, and on the other hand alkaline ions, this method being based on the above-mentioned discoveries and being characterizd in that the said alkaline ions comprise on the one hand Li and on the other hand at least one of the ions K and Na*.

According to another characteristic feature, the bath of molten salts according to the invention, advantageously contains, in addition to the cation Li at the same time the cations K and Na*, the respective proportions of the three cations Li K and Na being such that their combination with the anion CO terminates in a eutectic mixture of the three carbonates, the melting point of which at 397C, is very much lower than the working temperature of the bath, which is comprised between 450 and 500C. The remainder is constituted either by the anion CNO when it is desired to introduce nitrogen and carbon into the surface layers of the parts treated, or by the anions CNO' and sulphur anions such as for example anions chosen from S or S0 or again S 0 when it is desired to introduce nitrogen, carbon and sulphur into the superficial layers of the parts treated, or by the anions CNO, the phosphorus'anions P0 and S or S0 or alternatively S 0 when it is desired to introduce conjointly nitrogen, carbon, sulphur and phosphorus. It will be noted that the presence of the element phosphorus in the white superficial micro-layer increases resistance to corrosion of the parts thus treated.

In addition to a rate of growth of the micro-layer twice as high as that obtained with known baths, the presence of the cation Li has the effect of reducing considerably the consumption of salts which are carried away by the treated parts as they pass out of the bath. Another property of the bath according to the invention is that the contents of CNO and CO anions may vary in large proportions without involving any substantial modification of the characteristics of the treated parts, both as regards resistance to fatigue and resistance to wear. This especially enables the frequency of the checks and analyses of the bath to be considerably reduced.

The method according to the invention has the further advantage that the bath is practically free from cyanide, which avoids all risk of pollution. This is generally but more particularly the case when the bath contains ions.

The bath of molten salts according to the invention is further characterized in that the ratio by weight of Li to Na is comprised between 0.15 and 0.50 and the ratio by weight of Li to K is comprised between 0.10 and 0.35, the content by weight of anion CNO' in the bath being comprised between 20 and 65%, while the content by weight of anion CO of the bath is comprised between 1 and 35%.

In one preferred form of execution of the invention,

into a bath containing carbonate ions CO and cyanate ions CNO there is introduced a regenerating agent comprising urea (CO) (NI-[Q in order to convert the carbonate ions CO in excess to cyanate ions CNO so as to regenerate the said cyanate ions CNO. This generally makes it necessary to utilize canopies in order to trap the gaseous discharge which results.

' In order to prevent this evolution of gas and to dispense with such canopies, there is preferably introduced a regenerating agent comprising urea into the bath with a hermetic conduit opening into the body of the bath and having supply means of regenerating agent such as will result in this conduit being continuously isolated from the atmosphere.

This arrangement has especially the results:

a. An intimate mixture between the urea and the carbonate ions of the bath is obtained under the optimum conditions of treatment.

b. The carbon dioxide, ammonia and water vapour produced by the reaction between the urea and the carbonate ions are totally absorbed and dissolved in a bath of molten salts;

c. the evolution of gas takes place and is enclosed inside the conduit;

d. there is therefore a considerable reduction in the evolution of gas above the free surface of the bath in contact with the ambient atmosphere;

e. in the mass of molten salts there is induced an agitation which ensures greater homogeneity of the treatment bath.

In addition, in order to avoid all risk of choking-up the conduit with the regenerating agent, especially when the latter is introduced in powder form, means are advantageously provided for passing through the conduit towards the bath a gaseous conveying flow of the regenerating agent, while cooling means are preferably applied round the conduit in order to prevent the regenerating agent from sticking to the internal wall of the conduit.

The characteristic features and advantages of the method according to the invention will be brought out in the examples given below, without any implied limitation, the single FIGURE of the accompanying drawings illustrating in a diagrammatic manner an installation for carrying the method into effect, more particularly according to the first of the said examples.

EXAMPLE I Referring to the single FIGURE of the drawings, a vat 1 contains a bath B of molten salts comprising cyanates and carbonates for the treatment of metal surfaces, especially ferrous.

At the bottom of the vat l is arranged a diffuser means 2 comprising for example a strainer 3 provided with a large number of perforations 4. A conduit 5 dipping into the bath B has a lower outlet 6 arranged underneath the strainer 3 and enclosed by a cylindrical screen 7.'The conduit 5 is supplied from an upper inlet 8 by means of a dosing distribution device 9, with a regenerating agent in powder form, comprising in particular urea.

The apparatus 9 comprises a drum 10 in which is rotatably mounted, with a small clearance, a disc 11 provided with slots 12. The disc is driven in rotation by any appropriate driving means (not shown) for example a variable-speed motor-reduction-gear.

The drum 10 communicates at its upper portion with a hopper 13 which contains the regenerating agent, so that the slots 12 of the disc 11 are in contact with the regenerating agent at the level of the hopper and become filled-up as they pass.

The distribution device 9 thus extracts the regenerating agent by small quantities at regular intervals.

The conduit 5 is connected to the drum 10 by the inlet 8 and extends to the vicinity of this latter tangentially to the disc 11, while on the opposite side of the inlet 8 a conduit opens at 14 into the drum 10, this conduit 15 extending in the vicinity of the outlet 14, tangentially to the disc 11 and substantially in the line of extension of the inlet 8. Between the opening 14 and the inlet 8, the drum 10 has a bottom 16 which forms a small chamber 17 underneath the disc 11.

Around the conduit 5 is arranged in spaced-apart relation a jacket 18 which forms with the conduit 5 an annular space 19. The jacket 18 descends to the diffuser 2 and is coupled on the downstream side of the device 11 to a conduit 20.

Around the portion of the jacket 18 which dips into the bath is spaced apart a chimney 21 which forms with the jacket an annular space 22. The spaces 19 and 22 are completely isolated from each other, except at the lower portion in the vicinity of the strainer 3 where they communicate at 23. The chimney 21 is open at its upper extremity at 24.

The conduits 15 and 20 which comprise respectively constructions 25 and 26 are connected to a source of gaseous fluid under pressure 27 such as an air compressor.

In operation, the disc 11 is driven in rotation and the slots 12 take out doses of regenerating agent from the hopper 13, transfers them into the chamber 17 where they are projected by the flow of air from the conduit 15 into the conduit 5, and are brought under the strainer 3. The air passing out at 6 and the gases which result from the chemical reaction between the regenerating agent and the bath under the strainer, pass through the orifices 4 so as to cause an agitation in the bath B, while the chemical reaction continues in this bath.

At the same time, the flow of air passing through the conduit 20 is introduced into the annular space 19 and passes in the vicinity of the strainer 3 through the orifices 23 into the annular space 22, and is subsequently evacuated through the outlet 24 of the chimney 21.

This flow of air forms a thermal screen between the bath B and the conduit 5, and thus prevents any melting of the urea in powder which would be liable to cause blockages on the internal wall of the conduit 5. It will furthermore be appreciated that the flow of air passing through the conduit 5 and propelling the doses of regenerating agent ensures an effective conveyance of the agent and prevents any risk of obstruction of the conduit 5.

In the present example which has given good results, the bath B is obtained from a molten mixture of 16% by weight of potassium carbonate K CO 15% by weight of sodium carbonate Na,CO and 14% by weight of lithium carbonate Li,co,, while there is introduced into the bath by means of the device shown, 55% by weight of regenerating agent, which consists of a powder formed by urea (CO)/(Nl-l and of various fillers chosen in dependence on the results desired.

The bath B contained the following ions:

The temperature to which the above mixture was brought was comprised between 450and 600C, and waspreferably in the neighbourhood of 550C. The law of increase of the white micro-layer as a function of the immersion time, in the case where the treated part is of non-alloyed construction steel having a carbon content of 0.35% was very favorable. After only 30 minutes immersion, there was obtained a compact micro-layer of 20 microns in thickness, that is to say thicker than those obtained after minutes immersion in known baths.

It will be appreciated that the eutectic mixture permitted by the proportions indicated above has a melting temperature of 397C, that is to say a temperature substantially lower than that of the bath of 550C. This results in a great fluidity of the bath and prevents the immersed parts from carrying away a considerable quantity of salts when they are removed from the bath. Furthermore, it is not necessary to supervise the proportions of constituents in the bath at any great frequency, which provides great facility in the application and exploitation of the method.

EXAMPLE II The bath contained the following ions:

Li 4.0% K 14.8% Na 9.3% CO 26.9% CNO' 45.0%

The above mixture was brought to the same tempera ture as in Example 1, that is to say 550C. The law of increase of the white micro-layer as a function of the immersion time, in the case where the treated part is of non-alloyed construction steel having a carbon content of 0.35%, was identical with that obtained in Example 1, although the contents of CO and CNO' were in this case very different from those of Example 1.

EXAMPLE 111 The bath contained the following ions:

Li" 6.0% K 23.4% N21 14.0% C0," 16.1% CNO 40.0% S 0.5%

The above mixture was brought up to a temperature of 570C. After 60 minutes immersion in such a bath, parts of non-alloyed construction steel with 0.35% of carbon were coated with a white micro-layer having a base mainly of carbides, nitrides and sulphides of iron with a thickness in the neighbourhood of 30 microns.

The frictional performances of samples thus treated were very much greater than those obtained by treating identical samples in baths of carbon-nitro-sulphurization proposed up to the present and working at the same working temperature of 570C. In fact, a test sample treated as indicated above in the bath according to the invention and subjected to a standard test of the Faville type creeps under a load in the vicinity of 11,000 Newton, whereas an identical sample treated for 60 minutes in a known bath of carbo-nitro-sulphurization at 570C. creeps at a load comprised 5,000 and 7,000

Newton.

EXAMPLE IV The same procedure is followed as in Example 111, but the bath contained the following ions:

Li 4.5% K 14.4% Na" 1 1.3% CO," 16.1% CNO' 52.7% 803 It will be appreciated that with a bath of this kind, es pecially comprising as the sulphur ions the 80;, ions, the bath was, as in the other examples and more particularly in the present Example, substantially free of cyanide, which avoids all risk of pollution in the application of the method according to the invention.

EXAMPLE v The procedure is the same as for Example 111, but the bath contained the following ions:

1.1+ 5.5% 1 17.5% Na 13.3% C0," 16.1% CNO' 46.9% s,o,-- 0.7%

EXAMPLE VI The same procedure was followed as in the previous Examples, but the bath contained the following ions:

1.1 5.0% 10 18.5% Na+ 12.8% c0," 11.2% cNo- 50.0% s-- 0.5% P0; 2.0%

The above mixture was brought up to a temperature of 550C. After 60 minutes immersion in a bath of this kind, parts of non-alloyed construction steel with 0.35% of carbon were coated with a white micro-layer having a base mainly of carbides, nitrides, sulphides and phosphides of iron, having a thickness in the vicinity of 28 microns.

Test samples treated in this way in the bath according to the invention have a resistance to corrosion very much greater than that obtained by treating identical samples in known baths of carbo-nitro-sulphurization. In fact, after six months of atmospheric corrosion tests, the test samples treated in known baths are corroded over their entire surface, while the samples treated in the bath according to the invention have simply lost their brilliance; their weight remained unchanged for the whole duration of the corrosion tests.

What we claim is:

l. A method of treating machine parts of ferrous metals so as to form thereon a white microlayer of at least nitrides and carbides of iron, comprising the steps of immersing the parts to be treated in a bath of molten salts at a temperature between 450 and 600C. containing l-35% carbonate ions by weight, 20-65% cyanate ions by weight, potassium ions, sodium ions and lithium ions, the lithium being present in an amount about 4 to 6% by weight of the bath, the ratio by weight of lithium to sodium being 0.15 to 0.50 and the ratio by weight of lithium to potassium being 0.10 to 0.35, for a time sufficient to form a said white microlayer.

2. A method as claimed in claim 1, in which said bath contains a small but effective amount of sulphur ions selected from the group consisting of S, and S 0 said amount of sulphur ions being effective to introduce sulphur into said white microlayer.

3. A method as claimed in claim 1, in which said bath contains a small but effective amount of phosphate ions PO said amount of phosphate ions being effective to introduce phosphorus into said white microlayer.

4. A method as claimed in claim 1, and introducing said regenerating agent through a hermetic conduit 7 into said bath a regenerating agent for transforming into the bath through which a conveyor gas stream is carbonate ions into cyanate ions. circulated. v

5. A method as claimed in claim 4, said regenerating method as claimed in claim 1, in which said time agent being urea. is at least about 30 minutes.

6. A method as claimed in claim 4, and introducing 

1. A METHOD OF TREATING MACHINE PARTS OF FERROUS METALS SO AS TO FORM THEREON A WHITE MICROLAYER OF AT LEAST NITRIDES AND CARBIDES OF IRON, COMPRISING THE STEPS OF IMMERSING THE PARTS TO BE TREATED IN A BATH OF MOLTEN SALTS AT A TEMPERATURE BETWEEN 450* AND 600*C. CONTAINING 1-35% CARBONATE IONS BY WEIGHT, 20-65% CYANATE IONS BY WEIGHT, POTASSUM IONS, SODIUM IONS AND LITHIUM IONS, THE LITHIUM BEING PRESENT IN AN AMOUNT ABOUT 4 TO 6% BY WEIGHT OF THE BATH, THE RATIO BY WEIGHT OF LITHIUM TO SODIUM BEING 0.15 TO 0.50 AND THE RATIO BY WEIGHT OF LITHIUM TO POTASSIUM BEING 0.10 TO 0.35 FOR A TIME SUFFICIENT TO FORM A SAID WHITE MICROLAYER.
 2. A method as claimed in claim 1, in which said bath contains a small but effective amount of sulphur ions selected from the group consisting of S--, SO3-- and S2O3, said amount of sulphur ions being effective to introduce sulphur into said white microlayer.
 3. A method as claimed in claim 1, in which said bath contains a small but effective amount of phosphate ions PO3--, said amount of phosphate ions being effective to introduce phosphorus into said white microlayer.
 4. A method as claimed in claim 1, and introducing into said bath a regenerating agent for transforming carbonate ions into cyanate ions.
 5. A method as claimed in claim 4, said regenerating agent being urea.
 6. A method as claimed in claim 4, and introducing said regenerating agent through a hermetic conduit into the bath through which a conveyor gas stream is circulated.
 7. A method as claimed in claim 1, in which said time is at least about 30 minutes. 